Electron microscope with automatically adjusted specimen stage

ABSTRACT

A specimen stage for an electron microscope having a control system for automatically maintaining a selected point of the specimen in the center of the field of view, during tilting of the specimen. One disclosed control system comprises an electrical computing circuit, including potentiometers coupled to the stage for sensing tilting and the position of the point under observation. In another embodiment, the control system comprises an enlarged-scale mechanical model of the specimen stage, upon which measurements are made.

United States Patent [1 1 Page 5] Apr. 10, 1973 ELECTRON MICROSCOPE WITH[56] References Cited AUTOMATICALLY ADJUSTED D NT SPECIMEN STAGE UNITESTATES PATE S 2,494,442 1/1950 Le Poole ..250/49.5 A [75] InventRlchal'd Smckbndge Page, Great 3,240,934 3/1966 -Watanabe ..250 49.5 B

Dunnow, England [73] Assignee: iated Electrical Ind I PrimaryExaminerJames W. Lawrence Lo d E d Assistant Examiner-C. E. Church glanAttorney-Watts, Hoffmann, Fisher & l-leinke 22 Filed: June 22,1971 57ABSTRACT [21] Appl. No.: 155,474 1 A specimen stage for an electronmicroscope having a control system for automatically maintaining aForeign Application Priority Data selected point of the specimen in thecenter of the June 29, 1970 Great Britain ..31,456/70 field of Viewduring tilting the P" one t closed control system comprises anelectrical computing circuit, including potentiometers coupled to the[52] US. Cl. ..250/ 49.5 B stage for Sensing tilting and the position ofthe point nder b ervation In another embodimvent the con- [58] Fleld ofSearch ..250/49.5 B trol sy tem comprises an enlarged-scale mechanicalmodel of the specimen stage, upon which measurements are made.

15 Claims, 11 Drawing Figures PATENTEDAPRI e m 3.727, 051

SHEET 1m 5 Jam Fig.4

PATENTED 1 01975 3, 727. 051

sum 2 or 5 PATENTEI] APR 1 01973 SHEET 3 OF 5 Fig.5 S'n Singw Cos SinSinq5 Sinqb PATENTED APR 1 0191s SHEET UF 5 ELECTRON MICROSCOPE WITHAUTOMATICALLY ADJUSTED SPECIMEN STAGE I BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates generally to an electronmicroscope, and more particularly to an automatically adjusted specimenstage for an electron microscope.

2. Prior Art A specimen stage for an electron microscope may comprise agoniometer upon which the specimen can be mounted, and by means of whichthe specimen can be tilted about two mutually perpendicular axes withrespect to the optical axis of the microscope, so that details of thespecimen can be observed from different angles.

It is also known to provide specimen positioning controls for applyingtranslational movement to the goniometer in a plane transverse to theoptical axis. By this arrangement a selected point of the specimen canbe positioned on the optical axis, and therefore in the center of thefield of view of the microscope.

It will be appreciated that in general in such an arrangement thegoniometer axes do not intersect on the optical axis of the microscope.Hence, when the specimen is tilted the selected point of the specimenwill move off the optical axis; i.e. the field of view of the microscopewill be shifted. In order to maintain the same field of view,corrections must be applied to the specimen positioning controls as thespecimen is tilted, the correction usually requiring to be made in astepwise manner: i.e. first tilting the specimen slightly so that theobserved point does not leave the field of view, then adjusting thespecimen positioning controls to restore the observed point to thecenter of the field of view, then tilting the specimen slightly further,and so on until the desired tilt is obtained. This is clearly a slow andtedious process.

SUMMARY OF THE INVENTION The present invention overcomes the foregoingdrawbacks of the prior art, and provides a specimen stage whichautomatically adjusts the transverse position of the specimen duringtilting to maintain a selected point of the specimen on the optical axisof the microscope.

An electron microscope is provided with a specimen stage comprising: agoniometer for supporting a specimen and for tilting the specimenrelative to the optical axis of the microscope; positioning means forapplying translational movement to the goniometer in a plane transverseto the optical axis so as to enable a selected point of the specimen tobe positioned on the optical axis; a tilt representing means responsiveto the tilt applied to the specimen by the goniometer; positionrepresenting means responsive to the position in the specimen of saidselected point; computing means fed by said tilt and positionrepresenting means to produce an output representative of a correctivetranslational movement to be applied to the goniometer in order to holdsaid selected point of the specimen substantially on the optical axiswhile said tilt is applied to the specimen; and means for applying saidoutput to said stage positioning means so as to correct thetranslational position of the goniometer.

In accordance with another aspect of the present invention, thecomputing means is also arranged to produce a focal length adjustmentsignal which is representative of the change in focal length of the ob-5 jective lens of the electron microscope necessary to hold saidselected point of the specimen substantially in focus while said tilt isapplied to the specimen.

In one embodiment, the computing means comprises an electrical computingcircuit which may be either digital or analog.

The tilt and position representing means comprise potentiometers coupledmechanically to the goniometer and to the specimen positioning controls.

In another embodiment, the computing means comprises a mechanical modelof the goniometer and specimen, preferably on substantially enlargedscale. The tilt, and the position of said selected point in the specimenare simulated by mechanical displacement of corresponding parts of themodel, the corrective translational movement being derived from amechanical displacement of another part of the model. In this case saidtilt and position representing means comprise mechanical linkagescoupling the model mechanically with the goniometer. Alternatively, saidtilt representing means may comprise a plurality of potentiometerscoupled to the goniometer, which potentiometers are arranged to controlelectric motors which control the mechanical configuration of the model.

Accordingly, it is the principal object of the present invention toprovide an electron microscope with an automatically adjustable specimenstage.

BRIEF DESCRIPTION OF THE DRAWINGS This and other objects and advantagesof the invention will become apparent from the following description ofpreferred embodiments of the invention, as read in conjunction with theaccompanying drawings in which:

FIG. 1 is a schematic sectional view of an electron microscope;

FIG. 2 is a perspective view of a specimen cartridge for the microscope,on an enlarged scale compared with FIG. 1;

FIG. 3 is a perspective view of the specimen cartridge of FIG. 2 mountedin the specimen stage of the microscope;

FIG. 4 is a vector diagram defining certain angles referred to;

FIGS. 5 9 are block circuit diagrams of computing means for the stage;

FIG. 10 is a schematic diagram of another part of the stage; and

FIG. 11 is a diagrammatic view of part of alternative computing meansfor the stage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 11, theelectron microscope comprises generally tubular evacuable housing 101 atone end of which is mounted an electron gun 102 for producing a beam ofelectrons directed along the axis of the housing 101. The electrons passin turn through a pair of condenser lenses 103, 104, an objective lens105, and two projector lenses 106, 107, and finally strike a fluorescentscreen 108 positioned in a viewing chamber 109, which is provided withan observation window 110. Each of the lenses 103 107 comprises awinding 111 and a magnetic yoke 112 having a bore 113 through which theelectron beam passes. The yoke 112 has a gap 114, and it is the magneticfield across this gap which produces the focussing action of the lens.The lenses 103 107 are substantially coaxial, their common axis ofsymmetry being the optical axis of the electron microscope.

A vacuum manifold 115 is provided for evacuating the interior of thehousing 101 and the viewing chamber 109.

The specimen stage of the microscope includes a specimen support 117,adapted to receive a specimen cartridge 116, which can be inserted intothe support by means of a specimen loading mechanism 118. When thecartridge 116 is inserted into the specimen stage, a specimen mountedwithin the cartridge, as will be described, is supported within the gap114 of the objective lens 105.

Referring now to FIG. 2, the specimen stage is as described inco-pending US. Pat. application, Ser. No. 145,694. The specimencartridge 116 is generally tubular in shape, and carries at its lowerend a goniometer assembly on which the specimen is mounted. Thegoniometer assembly comprises an outer gimbal ring 119, pivotallymounted within the cartridge 116, and an inner gimbal ring 120,pivotally mounted within the outer gimbal ring 119, the axis 121 of theouter gimbal ring being perpendicular to the axis 122 of the innergimbal ring. A specimen holder 120a can be mounted within the inner ringby means of a circlip.

The cartridge 1 16 carries a pair of pushrods 123, 124 which slide ingrooves in the upper end of the cartridge. These pushrods driverespective pulley wheels (not shown) pivotally mounted within thecartridge, which pulley wheels are coupled respectively to the inner andouter gimbal rings by means of drive wires (not shown). Thus, by slidingthe pushrods 123, 124 in these grooves the gimbal rings 119, 120 aretilted about their axes, and therefore the specimen is tilted withrespect to the optical axis of the microscope. The angles of tilt aboutthe axes 121, 122 will be referred to as and 4; respectively, where 6and d) are both zero when the rings 119, 120 both lie in a plane normalto the optical axis of the microscope.

Referring now to FIG. 3, when the specimen cartridge 116 is mountedwithin the specimen support 117, the ends of the push rods 123, 124 abutrespectively against the faces of toothed sectors 125, 126 pivotallymounted on the support 117, the rods being biassed against these facesby means of a pair of return springs (not shown). In order to apply tiltto the specimen about the axes 121, 122 of the gimbals, drive is appliedfrom electric motors M0. M,,,, mounted externally of the microscopehousing 101, via respective drive shafts 127, 128, to worms 129, 130which mesh respectively with the sectors 125, 126, thus causing thesectors 125, 126 to revolve, causing the push rods 123, 124 to slide intheir grooves.

The specimen support 117, carrying the cartridge 116, is mounted so asto be slidable to a limited extent with respect to the housing 101 in aplane transverse to the electron optical axis. This movement can beeffected by means of two electric motors Mx, My, positioned externallyof the housing 101, these motors driving threaded rods 131, 132 whichengage in threaded holes in the housing 101, and act directly on thesupport 117. Return motion is supplied by means of return springs (notshown). The drive shafts 127, 128 are universally and telescopicallyjointed so as to allow for this transverse motion of the support.

As well as the electric motors Mx, My, M0, M manual controls (not shown)are provided for moving the support 117 transversely and for tilting thespecimen, as will be described below.

It will be seen that by means of the rods 131, 132, the specimen can bepositioned so that any selected point of the specimen will be on theoptical axis, and therefore in the center of the field of view of thescreen 108. Tilt can then be applied to the specimen by means of thedrive shafts 127, 128, so that the selected point of the specimen can beviewed from different angles.

In the following description, a system of orthogonal coordinate axes X YZ will be used, the axes being fixed v with respect to the specimensupport 117, and having their origin at the point where the axes 121,122 of the goniometer intersect. It will be appreciated that, ingeneral, since the support 117 is moveable in a transverse plane withrespect to the optical axis this point does not lie on the optical axis.The Z-axis is parallel to the (i.e. axis the X-axis is parallel to theaxis 121 Le, the 0 -axis) and the Y-axis is therefore parallel to thedirection of the axis 122 (i.e. the da -axis) when 0 0.

Any selected point of the specimen can be defined by its coordinates (x,y, z,,) with respect to the X, Y, Z axes before tilt is applied to thespecimen. It is assumed that this selected point initially lies on theoptical axis of the microscope. After a tilt 6, 4; has been applied tothe specimen, this point of the specimen will, in general, have shiftedaway from the optical axis, and will have new coordinates (x, y, z) withrespect to the X, Y, Z axes, these coordinates being given by thevecwhere the transfornmtion matrix A is given by cos; 9 sin .1: sin 0sin qb sin 0 cos 4) cus 0 sin cos 6 cos qS cos 0 sin 0 2) Thus, in orderto return the selected point to the center of the field of view, it isnecessary to move the specimen positioning controls until the point(x,y,z) lies on the optical axis. To achieve this, the matrix A must beconstructed, and equation (1) must be solved, as will be describedbelow.

Referring now to FIG. 4, when an operator wishes to apply a tilt to aspecimen which he is examining under the microscope, he will usuallywish to specify the tilt in terms of the magnitude and direction of thetilt relative to the Z axis and is not directly interested in the tiltangles 0, (b as defined above. The magnitude of the tilt is defined bythe angle 1 which is the angle between n, the normal to the surface ofthe specimen, and the Z cos @sin I ---(3) sin cos d) sinsin I 4Referring now to FIGS. and 6, the computing circuit of the specimenstage allows the operator to set the values of H and D, which hedesires, and automatically computes the corresponding values of theangles 0 and d), applying these angles to the goniometer. The part ofthe computing circuit shown in FIGS. 5 and 6 also computes the values ofthe elements of the matrix A.

The values ofand D are set by the operator by positioning the shafts oftwo sine-law potentiometers 36,

- 37. The ends of potentiometer 36 are supplied with respective unitvoltages,'positive at one end and negative at the other, and. a signalproportional to sin 4 is thus derived from the potentiometer 37 at point9. This signal is fed via a phase splitter 38 to the potentiometer 36,from which voltages are thus derived at points 10, 11 which areproportional to sinsin I and cossin I respectively.

To the drive shaft 127 which tilts the outer gimbal ring 1 19 (i.e. thatcontrolling the tilt angle 0) is coupled a set of three sine-lawpotentiometers 39, 41), 41, while to the drive shaft 128 which tilts theinner gimbal ring 120 (i.e. that controlling the tilt angle 4)) iscoupled a single sine-law potentiometer 42.

The ends of the potentiometer 39 are supplied with respective unitvoltages, positive at one end and negative at the other, so that signalsproportional to sin 0 and cos 0 are derived from this potentiometer atpoints 1 and 2 respectively.

Similarly, the ends of potentiometer 42 are also supplied'withrespective unit voltages, positive at one end and negative at the other,so that signals proportional to sin (I) and cos (I) are derived fromthis potentiometer at points 7, 4 respectively.

The sin signal from point 7. is fed via a phase splitter 43 to thepotentiometer 40, from which are thus derived signals proportional tosin 6 sin 41 and cos 0 sin (I: at points 3 and 4 respectively.

The cos (1) signal from point 8 is fed via a phase splitter 44 to thepotentiometer 41, from which are derived signals proportional to sin 0cos do and cos 0 cos at points 5 and 6 respectively.

The signals from the points 5 and 10, being proportional to sin 0 cos d)and sin@ sin 1 respectively, are added together in a summing amplifier45, the output of which is fed via a signal processing network G0 to theelectric motor M which drives the6 drive shaft 127. Similarly thesignalsfrom points 7 and 11, proportional to sin and cos sin 1respectively, are subtracted in a differential amplifier 46, the outputof which is fed via a processing network G9 to the electric motor Mwhich drives the (I) drive shaft 128.

The motors My, 1% operate to adjust thetilt angles 0 and 6 until theoutputs of the amplifiers 45, 46

are both zero, and it will be seen that in this condition the equations(3) and (4) are both satisfied. Thus the circuitry so far describedeffectively solves the equations (3) and (4) and applies to the specimena tilt corresponding to the anglesand 1 set by the operator.

It will be seen that the values of the eight non-zero elements of thematrix A, are given respectively by the voltages at the points 1 to 8.

Referring now to FIGS. 7, 8 and 9, the vector equation l) is solved bythe part of the computing circuit shown in these figures.

The values of x,,, y z,, are set by the operator by means of threeshafts, 201, 202, 203 each of which carries a set of linearpotentiometers 204-21 1 fed from the points 11-8, as shown in FIGS. 5and 6, by way of phase splitters 212 219, so as to multiply together theelements of A with the appropriate components of the vector (x,,, y,,, z

The X and Y specimen positioning control rods 131, 132 have linearpotentiometers 47, 48 respectively, at tached to them, a unit voltagebeing applied to each end of each of these, positive at one end andnegative at the other, so that signals are obtained from thepotentiometers 47, 48, at points 23, 24, proportional respectively to xand y, the X and Y co-ordinates of the point which actually lies on theoptical axis.

The voltages at points 12 and 19 are summed in an operational amplifier49 in which the voltage at point 23 is, at the same time, subtractedfrom the sum, the output from the amplifier 49 being fed, via a signalprocessing network G to the electric motor M which drives the rod 131 ofthe specimen positioning control.

Similarly, the voltages at points 13, 16 and 20 are summed in anoperational amplifier 50, and the voltage from the point 24 issubtracted from the sum, the output from the amplifier 50 being fed, viaa signal processing network Gy, to the electric motor My which drivesthe rod 132 of the specimen positioning control.

It will be seen that the motors M M position the specimen so that theoutputs of the amplifiers 49, 50 are both zero, and in this position xand y both satisfy equation l with the result that the selected point ofthe specimen (i.e. the point with co-ordinates (x,,2 y,,, z,,) beforetilting) is held substantially in the center of the field of view of themicroscope while the tilt 6, q: is applied.

The voltages at the points 14, 17 and 21 are summed in operationalamplifier 51, and from this sum is subtracted a feedback signal from theobjective lens current control 52, from the point 25, this signal beinga measure of z, the Z co-ordinate of the point at the center of thefield of view. The output from the amplifier 51 is thus a measure of theamount of correction to be applied to the focal length of the objectivelens in order to keep the selected point in focus, and it is accordinglyapplied to the objective lens control 52.

One further potentiometer 220-222 is attachedto each of the x,,, y,,,and 2,, shafts (see FIG. 3) and is fed with a unit voltage at each end(positive atone end, negative at the other). Thus, signals proportionalto 1: y, and z, are obtained from these potentiometers, at points 15, 18and 22 respectively, and can be used in a display device to enable theoperator to monitor the coordinates of the selected point. Further means(not shown) may be provided for displaying the values of@ and 4 to theoperator. Such a display might take the form of a vector diagram.

Referring now to FIG. 10, the four drives, for the 6 and q tilts and forthe X and Y movements, are provided with'manual controls as well as theautomatic controls M0. M M and M FIG. shows the arrangement of the twosets of controls for the 0 tilt movement, but it will be understood thatsimilar arrangements are provided for the other three controls.

In order to ensure that only one of the controls, either the automaticor the manual, is applied at a time to the drive shaft which controlsthe tilt 6, the drives from the automatic control M9, and the manualcontrol 53, after being geared down by gearboxes G1 and G2 respectively,are coupled via clutches C1 and C2 respectively to an output gearbox G3.The clutches C1 and C2 are electromechanically operated by means ofcoils 54, 55, by means of an actuating circuit (not shown) which is soarranged that only one of the coils can be energized at a time.

The set of potentiometers 39, 40, 41 which measure the angle 6, iscoupled to the gearbox G3 so that it operates independently of which ofthe clutches C1, C2 is engaged.

It will be appreciated that the accuracy with which the correctivetranslational motions are applied to the goniometer depends largely onthe accuracy of the components of the computing circuit, especially thepotentiometers, and therefore these should all be of the highestpossible accuracy.

The computing circuits described above are designed for the double tiltstage shown in FIGS. 2 and 3 or for a stage kinematically equivalent tothat stage, and it will be appreciated that if a stage of differentkinematical design were used, a different computing circuit would berequired.

It will be appreciated that a digital computer could be used instead ofthe analog computing circuits described above, in which case digitalcoding devices would be required to feed the values of I 0, x,,, y,,,z,,, x, y and 1 into the computer. The computer would be programmed toapply the correct tilts and translational movements to the specimen inaccordance with equations l (4).

One advantage of using a digital computer would be that the programcould easily be changed to allow the microscope to be used with a stageof different kinematical design.

Referring now to FIG. 11, in an alternative arrangement the computingmeans comprises a mechanical model of the goniometer and the specimen.The model is on a larger scale than the actual goniometer, beingtypically 30 times the size.

The model comprises a horizontally fixed support frame 60 within whichis mounted a gimbal frame 61 rotatable about an axis 62 with respect tothe support frame 60, and a table 63 mounted within the frame 61 androtatable about an axis 64 with respect to the frame 61.

Thus, the model is of identical kinematical design to the actualgoniometer. The frame 61 and the table 63 are tilted through angles 0and I by means of electric motors, MH and M,;,' which are fed with thesame tilting signals as are the servomotors M0 and M,,, which tilt thegoniometer. Thus, the table 63 automatically has the same tilt appliedto it as the specimen. A carriage 65 is mounted on the table 63 and canbe driven to any point on the table by means of electric motors M M,also mounted on the table. The position of the carriage on the table 63represents the position of the point of the specimen which is selectedfor observation.

A rod 66 is fixed at one end to the carriage 65 by means of a ball joint67. At the otherend, the rod 66 is fixed to a further carriage 68 bymeans of a double frame joint 69 which is also of the same kinematicaldesign as the goniometer. The rod 66 is free to move lengthwise in thejoint 69. The carriage can be moved in a plane parallel to the plane ofthe support frame 60, by means of a pair of electric motors M M,, whichalso serve to drive the specimen positioning controls of the actualgoniometer, through reduction gearing.

On the axes of the joint 69 are provided a pair of potentiometers (notshown) which measure angles p., v which specify the inclination of therod 66 to the vertical. Signals from these potentiometers are used asreference signals for the motors M,, M such that these servomotors actto keep the rod 66 vertical if the table 63 is tilted.

The lateral displacement of the carriage 68 required to keep the rodvertical is equal to the lateral displacement of the carriage 65 whenthe table 63 is tilted. Since the motors M M, are also coupled to theactual specimen stage, it will be seen that they automatically apply anappropriate translational correction to the actual goniometer in orderto hold the selected point of the specimen on the optical axis of themicroscope.

In operation of the microscope, the model is first of all set so thatthe table 63 is horizontal, i.e. in the plane of the support frame 60.Signals are then applied to the servomotors M M,, representative of theco-ordinates of the point of interest in the specimen. The carriage 65thus assumes a position on the table 63 which represents the selectedpoint of the specimen, and the carriage 68 moves a automatically to keepthe rod 66 vertical. Simultaneously, the actual goniometer is moved byservomotors M M In this way, the selected point is positioned on theoptical axis.

A tilt can now be applied to the specimen, the same tilt beingautomatically applied to the table 63. The carriage 68 again moves so asto keep the rod 66 vertical, and the corrective translational movementis automatically applied to the goniometer by the servomotors It will beappreciated that while the input signals applied to the model in FIG. 11are all electrical, in some cases it may be possible to couple the modelmechanically to the goniometer, in which case the input signals would bepurely mechanical.

It will be appreciated that the circuits shown in FIGS. 5 and 6, forconverting theand I settings into 0 and qb tilts, could be used withoutthe circuits in FIGS. 6, 7, 8 and 9 for applying automatic correctivetranslational movement to the goniometer.

In such a specimen stage, with no automatic corrective movement, thecircuits shown in FIGS. 5 and 6 may be simplified as follows.

Referring to FIG. 4, if the magnitude of the tilt, defined by the angleD is reasonably small, equations (3) and (4) can be considerablysimplified, by making the approximation that I is equal to sin 1 Thus:

sin I sin@ 4" Finally, it is evident from (4") that 0 is also small, sothat we can make a further approximation:

Thus, in the simplified arrangement, the potentiometers 37, 39 and 42 inFIGS. 5 and 6 can all be replaced by linear potentiometers which aremuch less expensive than the sine or cosine type. In addition, sincethere is no automatic translational correction, the potentiometers 40and 41, as well as the circuits shown FIGS. 7, 8 and 9, are not neededat all. Translational correction to maintain the field of view when tiltis applied, must be made manually in this case.

It will be appreciated that the invention may be applied to the scannedtransmission type of electron microscope as well as to the non-scannedtype.

Although the invention has been shown in connection with preferredembodiments, it will be readily apparent to those skilled in the artthat various changes in the form and arrangement of parts may be made tosuit requirements without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is: p

1. In an electron microscope comprising a source of electrons forproducing a beam of electrons directed generally along an electronoptical axis, an image screen, a specimen stage for supporting andcontrolling the position of a specimen in said beam, and an electronoptical system for focussing said beam to form an electron image of aportion of said specimen on said screen, said specimen stage comprising:

a goniometer for supporting the specimen and for tilting the specimenrelative to the electron optical axis;

positioning means for applying translational movement to the goniometerin a plane transverse to the electron optical axis so as to enable aselected point of the specimen to be positioned on the electron opticalaxis for viewing on said screen;

tilt representing means, responsive to the tilt applied .to the specimenby the goniometer;

position representing means, responsive to the position in the specimenof said selected point;

computing means fed by said tilt and position representing means so asto produce an output representative of a corrective translationalmovement to be applied to the goniometer in order to hold said selectedpoint substantially on the optical axis while said tilt is applied tothe specimen;

- and means for applying said output to said positioning means so as tocorrect the translational position of the goniometer.

2. A specimen stage according to claim 1 wherein said computing meanscomprises means for producing a focal length adjustment signal,representative of the change in focal length of the objective lens ofthe electron microscope necessary to hold said selected point of thespecimen substantially in focus while said tilt is applied to thespecimen, and said specimen stage comprises means for applying saidfocal length adjustment signal to the objective lens.

3. A specimen stage according to claim 1 wherein said computing meanscomprises a digital electrical computing circuit.

4. A specimen stage according to claim 1 wherein said computing meanscomprises an analog electrical v computing circuit.

5. A specimen stage according to claim 4 wherein said tilt representingmeans comprises a plurality of potentiometers coupled mechanically tothe goniometer.

6. A specimen stage according to claim 4 wherein said positionrepresenting means comprises a plurality of potentiometers, coupled tothe specimen positioning controls.

7. A specimen stage according to claim 1 wherein said computing meanscomprises:

a mechanical model of the goniometer and specimen, said tilt and theposition of said selected point in the specimen being simulated bymechanical displacement of corresponding parts of the model; and

means for deriving said output representative of a correctivetranslational movement from a mechanical displacement of another part ofthe model. i

8. A specimen stage according to claim 7 wherein said model is on asubstantiallyenlarged scale compared with the goniometer.

9. A specimen stage according to claim 7 wherein said tilt representingmeans comprises a plurality of potentiometers coupled to the goniometer;

and said computing means includes a plurality of electric motors coupledmechanically to said model and coupled electrically to saidpotentiometers.

10. A specimen stage according to claim 7 wherein the model comprises:

a table, so mounted on a support that it can be tilted in kinematicallythe same manner as the specimen;

a carriage mounted on the table and moveable upon the table, theposition of the carriage on the table representing the position of theselected point in the specimen;

and means for measuring lateral displacement of the carriage withrespect to the support when said tilt is applied to the table, and forproducing said output representative of said corrective translationalmovement.

11. A specimen stage according to claim 10 wherein said means formeasuring the lateral displacement comprises;

a rod universally coupled at one end to the carriage and at the otherend universally coupled to a further carriage moveable in a plane fixedwith respect to the support, the rod being free to move lengthwise insaid further carriage; and

means for moving said further carriage in its plane so as to maintainsaid rod in a fixed orientation with respect to the support;

the movement of said further carriage being thus proportional to saidlateral displacement.

12. A specimen stage according to claim I, further including;

setting means for setting the values of a required magnitude anddirection of tilt to be applied to said specimen by the goniometer withrespect to said electron optical axis;

control means for computing the angles of tilt about the goniometer axesrequired to produce the required magnitude and direction of tilt;

and means for applying the computed angles of tilt to the goniometeraxes.

13. In an electron microscope comprising a source of electrons forproducing a beam of electrons directed generally along an electronoptical axis, an image screen, a specimen stage for supporting andcontrolling the position of a specimen in said beam, and an electronoptical system for focusing said beam to form an electron image of aportion of said specimen on said screen, said specimen stage comprising:

a. specimen mounting means for adjustably tilt-ably mounting a specimenwithin the beam of electrons at selected orientations with respect tothe electron optical axis;

b. positioning means for applying translational movement to saidspecimen mounting means in a plane transverse to the electron opticalaxis to enable a selected point of the specimen to be positioned on theelectron optical axis for viewing on the screen; and,

c. actuating means coupled to said specimen mounting means forautomatically actuating said positioning means in response to changes inorientation of the specimen with respect to the electron optical axis tomaintain a point under observation substantially along said optical axisduring tilting.

14. The specimen stage of claim 13 wherein said actuating meanscomprises sensing means coupled to said specimen mounting means forsensing changes of orientation of the specimen with respect to theelectron optical axis, and computing means coupled to said sensing meansfor producing an output representative of a translational movement to beapplied to said specimen mounting means and for applying said output tosaid positioning means.

15. The specimen stage of claim 13, further including means coupled tosaid specimen mounting means for automatically adjusting said electronoptical system in response to changes of orientation of the specimenwith respect to the electron optical axis to maintain a point underobservation substantially in focus on said screen during tilting.

1. In an electron microscope comprising a source of electrons forproducing a beam of electrons directed generally along an electronoptical axis, an image screen, a specimen stage for supporting andcontrolling the position of a specimen in said beam, and an electronoptical system for focussing said beam to form an electron image of aportion of said specimen on said screen, said specimen stage comprising:a goniometer for supporting the specimen and for tilting the specimenrelative to the electron optical axis; positioning means for applyingtranslational movement to the goniometer in a plane transverse to theelectron optical axis so as to enable a selected point of the specimento be positioned on the electron optical axis for viewing on saidscreen; tilt representing means, responsive to the tilt applied to thespecimen by the goniometer; position representing means, responsive tothe position in the specimen of said selected point; computing means fedby said tilt and position representing means so as to produce an outputrepresentative of a corrective translational movement to be applied tothe goniometer in order to hold said selected point substantially on theoptical axis while said tilt is applied to the specimen; and means forapplying said output to said positioning means so as to correct thetranslational position of the goniometer.
 2. A specimen stage accordingto claim 1 wherein said computing means comprises means for producing afocal length adjustment signal, representative of the change in focallength of the objective lens of the electron microscope necessary tohold said selected point of the specimen substantially in focus whilesaid tilt is applied to the specimen, and said specimen stage comprisesmeans for applying said focal length adjustment signal to the objectivelens.
 3. A specimen stage according to claim 1 wherein said computingmeans comprises a digital electrical computing circuit.
 4. A specimenstage according to claim 1 wherein said computing means comprises ananalog electrical computing circuit.
 5. A specimen stage according toclaim 4 wherein said tilt representing means comprises a plurality ofpotentiometers coupled mechanically to the goniometer.
 6. A specimenstage according to claim 4 wherein said position representing meanscomprises a plurality of potentiometers, coupled to the specimenpositioning controls.
 7. A specimen stage according to claim 1 whereinsaid computing means comprises: a mechanical model of the goniometer andspecimen, said tilt and the position of said selected point in thespecimen being simulated by mechanical displacement of correspondingparts of the model; and mEans for deriving said output representative ofa corrective translational movement from a mechanical displacement ofanother part of the model.
 8. A specimen stage according to claim 7wherein said model is on a substantially enlarged scale compared withthe goniometer.
 9. A specimen stage according to claim 7 wherein saidtilt representing means comprises a plurality of potentiometers coupledto the goniometer; and said computing means includes a plurality ofelectric motors coupled mechanically to said model and coupledelectrically to said potentiometers.
 10. A specimen stage according toclaim 7 wherein the model comprises: a table, so mounted on a supportthat it can be tilted in kinematically the same manner as the specimen;a carriage mounted on the table and moveable upon the table, theposition of the carriage on the table representing the position of theselected point in the specimen; and means for measuring lateraldisplacement of the carriage with respect to the support when said tiltis applied to the table, and for producing said output representative ofsaid corrective translational movement.
 11. A specimen stage accordingto claim 10 wherein said means for measuring the lateral displacementcomprises; a rod universally coupled at one end to the carriage and atthe other end universally coupled to a further carriage moveable in aplane fixed with respect to the support, the rod being free to movelengthwise in said further carriage; and means for moving said furthercarriage in its plane so as to maintain said rod in a fixed orientationwith respect to the support; the movement of said further carriage beingthus proportional to said lateral displacement.
 12. A specimen stageaccording to claim 1, further including; setting means for setting thevalues of a required magnitude and direction of tilt to be applied tosaid specimen by the goniometer with respect to said electron opticalaxis; control means for computing the angles of tilt about thegoniometer axes required to produce the required magnitude and directionof tilt; and means for applying the computed angles of tilt to thegoniometer axes.
 13. In an electron microscope comprising a source ofelectrons for producing a beam of electrons directed generally along anelectron optical axis, an image screen, a specimen stage for supportingand controlling the position of a specimen in said beam, and an electronoptical system for focusing said beam to form an electron image of aportion of said specimen on said screen, said specimen stage comprising:a. specimen mounting means for adjustably tiltably mounting a specimenwithin the beam of electrons at selected orientations with respect tothe electron optical axis; b. positioning means for applyingtranslational movement to said specimen mounting means in a planetransverse to the electron optical axis to enable a selected point ofthe specimen to be positioned on the electron optical axis for viewingon the screen; and, c. actuating means coupled to said specimen mountingmeans for automatically actuating said positioning means in response tochanges in orientation of the specimen with respect to the electronoptical axis to maintain a point under observation substantially alongsaid optical axis during tilting.
 14. The specimen stage of claim 13wherein said actuating means comprises sensing means coupled to saidspecimen mounting means for sensing changes of orientation of thespecimen with respect to the electron optical axis, and computing meanscoupled to said sensing means for producing an output representative ofa translational movement to be applied to said specimen mounting meansand for applying said output to said positioning means.
 15. The specimenstage of claim 13, further including means coupled to said specimenmounting means for automatically adjusting said electron optical systemin response to changes of orientation of the specimen with respect tothe eleCtron optical axis to maintain a point under observationsubstantially in focus on said screen during tilting.